Identification of a co-activator that links growth factor signalling to c-Jun/AP-1 activation

The AP‑1 transcription factor c‑Jun is essential for cellular proliferation in many cell types, but the molecular link between growth factors and c‑Jun activation has been enigmatic. In this study we identify a previously uncharacterized RING-domain-containing protein, RACO‑1 (RING domain AP‑1 co-activator‑1), as a c‑Jun co-activator that is regulated by growth factor signalling. RACO‑1 interacted with c‑Jun independently of amino‑terminal phosphorylation, and was both necessary and sufficient for c‑Jun/AP‑1 activation. Growth factor-mediated stimulation of AP‑1 was attributable to MEK/ERK-dependent stabilization of RACO‑1 protein. Stimulation of the MEK/ERK pathway strongly promoted Lys 63-linked ubiquitylation of RACO‑1, which antagonized Lys 48-linked degradative auto-ubiquitylation of the same Lys residues. RACO‑1 depletion reduced cellular proliferation and decreased expression of several growth-associated AP‑1 target genes, such as cdc2, cyclinD1 and hb-egf. Moreover, transgenic overexpression of RACO‑1 augmented intestinal tumour formation triggered by aberrant Wnt signalling and cooperated with oncogenic Ras in colonic hyperproliferation. Thus RACO‑1 is a co-activator that links c‑Jun to growth factor signalling and is essential for AP‑1 function in proliferation.